Experimental study on the disinfection of microbial contamination by the microwave radiation on a typical filter coated with absorbing material

. In the air-conditioning system, a high-efficiency air filter can guarantee the capture of dust and biological particles. However, the traditional filter material itself does not have the function of disinfection and self-purification. As a result, the traditional air filter might become a hotbed for the reproduction of the microorganisms. Carried by the airflow, these microorganisms can enter the indoor environment, which might cause secondary pollutions. A properly selected upstream filter can prevent the particulate pollution, while it is hard to prevent the biological pollution inside the air-conditioning system. The microwave radiation is a broad-spectrum and high-efficiency disinfection method because of its disinfection mechanism, penetrability and selective heating property. At present, the microwave disinfection technology is widely used in the disinfection process of food processing, but its application in the air-conditioning field is relatively rare. From this point of view, this paper proposes to apply the microwave disinfection technology to the filters of the air-conditioning system. First, the combination of the filter and absorbing material is proposed to improve its absorption capacity of the microwave. Then, the air-conditioning system experimental platform for microwave disinfection was established and experiments were carried out to explore the disinfection effect of the Staphylococcus aureus on the composite filter under the microwave radiation. Finally, the experimental results and their influencing factors are briefly analyzed.


Introduction
With the increasing improvement of people's living standards, air-conditioning systems are widely used to provide a comfortable indoor thermal and humid environment for buildings [1].While providing people with a comfortable indoor environment, the airconditioning system has a suitable temperature and humidity environment for the growth of microorganisms [2].Moreover, the cleaning and disinfection inside the air-conditioning system cannot be guaranteed, and it is easy to breed microbial contamination.Therefore, if the air-conditioning system is improperly designed, operated or managed, it is likely to become a breeding ground for microorganisms.And the microorganisms inside the air-conditioning system can enter the indoor environment through the ventilation system, thereby polluting the indoor air and endangering the health of indoor personnel [2].
Most of the microorganisms in the air and airconditioning systems are attached to the dust particles and exist in the form of biological particles [3].Therefore, filtration is an effective measure to remove microbial contamination, and it is also the most widely used pollution control measure in the air-conditioning systems today.However, the traditional filter material itself does not have the function of disinfection and selfpurification.As a result, the traditional air filter might * Corresponding author: ffwang@hust.edu.cnbecome a hotbed for the reproduction of the bacteria, fungi, virus or other microorganisms, which will cause serious secondary pollution [4].Currently, a properly selected upstream filter can largely prevent particulate pollution, however, preventing biological contamination inside the air-conditioning systems remains challenging.Therefore, many scholars at home and abroad propose to use the disinfection devices or materials in combination with filters to kill microorganisms while intercepting them.For example, chitosan [5], grapefruit seed extract [6], electrostatic field [3], photocatalysis [4], etc.However, these methods have certain limitations in practical application.
The microwave radiation is a broad-spectrum and high-efficiency disinfection method because of its disinfection mechanism, penetrability and selective heating property [7].Many scholars at home and abroad have studied its disinfection effect in different application scenarios.Wu et al. [8] studied the survival rate of the microbial aerosols in laboratory culture and the natural environment under microwave radiation.Wang et al. [9] compared the disinfection effect, energy utilization and inactivation mechanism of the microwave radiation on E. coli in air and water.Lv et al. [7] studied the disinfection effect of the microwave radiation on the fungal spores in an evaporative humidifier through experiments.Wang et al. [10] used the sol-gel method to coat TiO2 on the Al2O3 filter material to intercept microorganisms, and used microwave radiation to kill the intercepted microorganisms.Zhang et al. [11] proposed a nanofiber air filtration disinfection system based on microwave radiation to kill pathogenic bacteria in the air.At present, the application of the microwave disinfection technology in the air-conditioning field is relatively rare, so the research on its application method and mechanism in the field of air-conditioning is of great significance.
In order to solve the above problems, this paper proposes to apply the microwave sterilization technology to the filters of the air-conditioning system.In order to improve the absorptive capacity of the filter material on microwave radiation, this paper proposes to combine the absorbing material with the filter material, and experimentally studies the disinfect effect of the microwave radiation on the microbial contamination on the composite filter.

Description of the experimental 2.1 Combination of the filter and absorbing material
In order to make the absorbing material adhere to the filter fiber evenly, firstly, the absorbing material and adhesive are mixed evenly and the mixing ratio is 30 g of the absorbing material per 100 ml of the adhesive.Then, an atomizing spray gun is used to evenly atomize the mixed material to the surface and internal fiber of the filter, and the spraying amount of each filter is about 10g.In this paper, 1μm SiC powder was selected as the absorbing material, and water-based polyurethane resin (F0401 PU) was selected as the adhesive, and the polyester fiber filter cotton (AF-A) was selected as the filter material.The size of each filter was 200mm*200mm*3mm.

Experimental platform of the airconditioning system for microwave disinfection
In order to explore the disinfection effect of the microwave radiation on the microbial contamination of the composite filter, a laboratory-scale experimental platform of the air-conditioning system for microwave disinfection was built in this paper, and its schematic diagram is shown as Fig. 1.The experimental platform is mainly composed of the air-conditioning system, the microwave emission system and the corresponding test system.The air-conditioning system includes the filters, the fan, the frequency converter, the air mixing box and the metal ducts to realize the actual operating conditions of the air-conditioning system.The microwave emission system includes the microwave magnetron, the rectangular waveguide, the power supply, the power meter, the regulator and the shielding mesh (copper mesh) to transmit microwave and control the input power.The test system includes the air compressor, the atomizer, the composite filter, the operating door, the anemometers and the temperature and humidity recorder to test the disinfection effect and related experimental parameters.

Microwave disinfection experiment 3.1 Preparation of the microbial suspensions
In this paper, the Staphylococcus aureus was selected as the representative microorganism to study the disinfection effect of the microwave radiation on the microbial contamination of the composite filter.Staphylococcus aureus is the test bacteria specified in the disinfection technical specification, and it is the representative of the Gram-positive bacteria.First, the cryopreserved strains in laboratory were dissolved at room temperature, and then 1 ml of the dissolved bacterial liquid was transferred to 100 ml of the beef extract peptone liquid medium, and cultured under suitable conditions.Since the strains are in a dormant state for long-term cryopreservation, and the activity of the strains has a great influence on the experimental results, it is necessary to continuously transfer for 4 generations to restore the activity.After the strains were activated, 1 ml of the activated bacterial liquid was transferred to 100 ml of the beef extract peptone liquid medium, and cultured in a constant temperature shaker at 30°C and 160 rpm for 18 h.Then, the cultured bacterial liquid was poured into a 50 ml centrifuge tube and centrifuged at 7000 rpm for 10 min.After centrifugation, the supernatant was removed, and the sterilized water was added into the centrifuge tube for resuspension before centrifugation.The bacterial cells obtained after the second centrifugation were added with a certain amount of sterilized water to prepare the bacterial suspension with a certain concentration of 5*10 8 CFU/ml, to prepare for subsequent experiments.

The experimental procedures
The experimental procedure designed in this paper is mainly divided into four parts, followed by atomization of the bacterial suspension, microwave disinfection, sampling and culture, and result analysis.The atomization process of the bacterial suspension is to put the pre-configured bacterial suspension into the atomizing bottle.After the air compressor is turned on, the bacterial suspension will be atomized and sent to the air mixing box to be mixed evenly with the supply air, then passed through the pre-placed composite filter with the supply air.During this process, the Staphylococcus aureus in the supply air will be intercepted by the filter and evenly distributed on the front and back surfaces and inside of the filter to prepare for the subsequent disinfection experiment.The microwave disinfection process is that after the atomization process is completed, turn on the microwave emitting device and run it at a specific power for a specific time (the control group does not turn on the microwave emitting device and let it stand for the same time to exclude the influence of natural reproduction and death of microorganisms on the experimental results).The sampling method selected in this paper is the contact dish sampling.This sampling method is widely used in the detection of microorganisms in clean environments.It is simpler and more accurate than the traditional cotton swab sampling method.In this paper, 4 measuring points are arranged on the front and back surfaces of the filter respectively, so there is a total of 8 measuring points, and their distribution diagram is shown as fig. 2. The specific sampling process is as follows.First, take out the contact dish to be used from the refrigerator and return to room temperature.Then open the top cover of the contact plate, hold the bottom of the contact plate and press the agar side to the part to be inspected, and press gently for 10 seconds.Finally, cover the contact dish and place it in a 37°C incubator for incubation.After culturing for 48 hours, the colonies growing on the contact plates of the control group and the experimental group were counted and the disinfection rate was calculated.Regardless of whether it is the control group or the experimental group, three replicate experiments were carried out for each condition.The formula for calculating the disinfection rate is: where R d is the disinfection rate, Ne and N c are the number of colonies in the experimental and control groups.

Results and discussion
Fig. 3 shows the disinfection rate of the staphylococcus aureus on the composite filter by the microwave radiation under different combination of the disinfection time and disinfection power.It can be seen from Fig. 3(a) that the disinfection rate of each measuring point can basically reach more than 90% after disinfection for more than 3 minutes under the disinfection power of 600w.Only when the disinfection time is 3 minutes, the disinfection rate of several measuring points in the upper half of the filter is slightly lower than 90%.When the disinfection time is 1min, the disinfection rate of each measuring point can reach more than 80%.For the measuring points in the upper half of the filter, when the disinfection time gradually increased from 1min to 8min, the disinfection rate also gradually increased, but when it increased from 8min to 10min, the disinfection rate did not increase significantly.For the measuring points in the lower half of the filter, when the disinfection time increased from 1min to 3min, the disinfection rate increased significantly, but when the disinfection time gradually increased from 3min to 10min, the disinfection rate did not increase significantly, and all reached more than 97%.This is mainly because the microwave radiation intensity around the lower half of the filter is higher than that of the upper half of the filter.However, there is no significant difference in the disinfection rate of the left and right sides and the front and back surfaces of the filter.This is mainly because the electromagnetic field is evenly distributed on the left and right sides of the disinfection section and has strong penetration, which can pass through the filter without being blocked by the filter, which is also an important feature of the microwave disinfection.Therefore, when the microwave radiation intensity is high, a good disinfection effect can be achieved in a very short time.
In general, under the disinfection power of 600w, the disinfection rate of over 90% can be achieved at each measuring point when the disinfect time is more than 5min.
It can be seen from Fig. 3(b) that when the disinfection power is reduced from 600w to 400w, the disinfection rate of each measuring point is significantly reduced.So the disinfection power is an important factor affecting the disinfection effect.For the measuring points in the upper half of the filter, when the disinfection time gradually increased from 1min to 5min, the disinfection rate also gradually increased, but when it increased from 5min to 10min, the disinfection rate did not increase significantly.The final disinfection rate is stable between 70%-80%.For the measuring points in the lower half of the filter, when the disinfection time gradually increased from 1min to 8min, the disinfection rate also gradually increased, but when it increased from 8min to 10min, the disinfection rate did not increase significantly, and the final disinfection rate was stable at about 90%.
It can be seen from Fig. 3(c) that when the disinfection power is reduced from 400w to 200w, the disinfection rate of each measuring point is further significantly reduced.When the disinfection time gradually increased from 1min to 8min, the disinfection rate of each measuring point gradually increased, and when it increased from 8min to 10min, the disinfection rate did not increase significantly, and the final disinfection rate was stable at around 40%.At this case, there is no significant difference in the disinfection rate of the upper and lower measurement points, this is mainly because the overall microwave radiation intensity in the disinfection section is weak at this case.
To sum up, increasing the disinfection power or disinfection time can improve the disinfection rate to a certain extent, and the influence of the disinfection power is more obvious.When the disinfection power is insufficient, even if the disinfection time is prolonged, a high disinfection rate cannot be achieved.And when the

Conclusions
In this paper, the disinfection effect of the microwave radiation on the Staphylococcus aureus on the filter coated with absorbing material was investigated experimentally.The results show that this disinfection device can effectively kill Staphylococcus aureus.Each measuring point can achieve a disinfection rate of more than 90% at 600w disinfection power for 5min.Within a certain range, increasing the disinfection power or prolonging the disinfection time can improve the disinfection rate to a certain extent, and the effect of increasing the disinfection power is more significant.

Fig. 2 .
Fig. 2. The distribution diagram of the measuring points.
sufficient, a high disinfection rate can be achieved in a very short disinfection time.

Fig. 3 .
Fig. 3.The disinfection rate under different combination of the disinfection time and disinfection power.